Depending on your inclination, a tractor beam of sound waves powerful enough to suspend you in mid-air is either a science fiction-inspired dream or a nightmare. Such a field, capable of moving heavy objects at will, is still a very long way off, but the main theoretical obstacle to such a device has just been lifted.

A tractor beam is a stream of energy that can move solid objects. It uses waves of either sound or light to transfer force across space. It's a common trope in sci-fi as a way for spaceships to grab smaller objects and reel them in.

Tractor beams already exist and are currently in use for blood analysis and chemistry, but only work on very small objects. The power requirements can be enormous, and this rises with the mass of whatever you are trying to move. An even more fundamental problem was that all attempts to trap objects larger than the size of the sound wave being used ended up causing targets to spin uncontrollably, rather than moving them as desired. A spinning object not only wastes applied energy, it is hard to guide and quickly gets ejected from the beam. The nausea-induction for a human caught in such a beam also doesn't bear thinking about.

Tractor beams within the range of human hearing would be very unpleasant to be around, let alone in, so experimental versions use frequencies beyond the 20,000 Hertz humans can typically detect. That has put severe limitations on the size of the objects they can shift, since the wavelength cannot exceed 17 millimeters (0.7 inches). However, Dr Asier Marzo of the University of Bristol has shown in Physical Review Letters these spin effects are avoidable.

Marzo and colleagues created acoustic vortices, which they compare to tornadoes of sound around a silent core. By frequently reversing the direction of the vortices, creating pulses that spin first one way, then the other, they can stabilize the beam, and with it the target. Already, they have broken the size barrier. Using 40kHz sound waves (a frequency popular with bats but inaudible to people) they have held a 1.7-centimeter (0.7-inch) polystyrene ball in the beam, despite it being double the size of their sound waves, and proved they could have gone larger still.

"Acoustic researchers had been frustrated by the size limit for years, so it's satisfying to find a way to overcome it. I think it opens the door to many new applications," Marzo said in a statement. So much energy is required don't expect carnival rides on tractor beams anytime soon, but experiments can begin on moderately sized objects.

By repeatedly reversing the direction of acoustic vortices around the target, an acoustic tractor beam can levitate an object substantially larger than the wavelength used. Colors indicate direction of spin. University of Bristol.